Conversion of a rotational movement of a driving means into an axial movement of a load is most frequently performed by means of drive mechanisms operating according to the screw-nut principle, i.e. with a threaded engagement between twp parts, one of which is axially movable, whereas the other is rotatable. For example, in a so-called rolling screw, threaded rollers which are kept in a roller holder are moved around a central threaded shaft having a thread inclination corresponding to that of the rollers. The roller holder will move along the shaft as an ordinary nut when being turned around the shaft and securing the shaft against rotation..
If the roller holder during rotation is secured against axial movement, the shaft will perform an axial movement. Conversely, if the shaft is rotated and the roller holder is secured against rotation, either the roller holder or the shaft will perform an axial movement when the other part is secured against axial displacement. Furthermore, if the system is not self-blocking in the axial direction, an axial movement of one part will be convertible into rotation of the other part.
Thus, the rolling screw has the same movement possibilities as an ordinary screw-nut connection, but considered as a transmission mechanism, it suffers in the same manner as the screw-nut connection of several considerable disadvantages. On one hand, it is very difficult to provide a simple axial sealing around a threaded shaft and, on the other hand, threaded shafts in greater lengths can only be produced at quite considerable expences. Furthermore, the transmission ratio of the mechanism will be determined once for all by the thread inclination, so that a change of the transmission ratio may only be obtained by a completely new conbination of a threaded shaft and a nut or rolling screw, whereby it will not be possible in this manner to obtain a gear mechanism having a stepless variable transmission ratio.